An electric power steering apparatus which provides a steering mechanism of a vehicle with a steering assist torque by means of a rotational torque of a motor, applies a driving force of the motor as the steering assist torque to a steering shaft or a rack shaft by means of a transmission mechanism such as gears or a belt through reduction gears. In order to accurately generate the steering assist torque, such a conventional electric power steering apparatus (EPS) performs feedback control of a motor current. The feedback control adjusts a voltage supplied to the motor so that a difference between a steering assist command value (a current command value) and a detected motor current value becomes small, and the adjustment of the voltage supplied to the motor is generally performed by an adjustment of duty command values of pulse width modulation (PWM) control.
A general configuration of the conventional electric power steering apparatus will be described with reference to FIG. 1. As shown in FIG. 1, a column shaft (a steering shaft or a handle shaft) 2 connected to a steering wheel 1 is connected to steered wheels 8L and 8R through reduction gears 3, universal joints 4a and 4b, a rack-and-pinion mechanism 5, and tie rods 6a and 6b, further via hub units 7a and 7b. In addition, the column shaft 2 is provided with a torque sensor 10 for detecting a steering torque of the steering wheel 1, and a motor 20 for assisting a steering force of the steering wheel 1 is connected to the column shaft 2 through the reduction gears 3. The electric power is supplied to a control unit (ECU) 100 for controlling the electric power steering apparatus from a battery 13, and an ignition key signal is inputted into the control unit 100 through an ignition key 11. The control unit 100 calculates a current command value of an assist (steering assist) command on the basis of a steering torque T detected by the torque sensor 10 and a vehicle speed V detected by a vehicle speed sensor 12, and controls a current supplied to the motor 20 on the basis of a current control value E obtained by performing compensation or the like with respect to the calculated current command value. Moreover, it is possible to receive the vehicle speed V from a controller area network (CAN) or the like.
In such an electric power steering apparatus, the control unit 100 mainly comprises a CPU (including an MPU and an MCU), and general functions performed by programs within the CPU are, for example, shown in FIG. 2.
The functions and the operation of the control unit 100 will be described with reference to FIG. 2. As shown in FIG. 2, the steering torque T from the torque sensor 10 is inputted into a current command value calculating section 101, and at the same time, is also inputted into a steering state judging section 120. The vehicle speed V from the vehicle speed sensor 12 is inputted into the current command value calculating section 101, and at the same time, is also inputted into a vehicle speed sensitive gain section 123. A current command value Iref calculated on the basis of the steering torque T and the vehicle speed V in the current command value calculating section 101 is addition-inputted into a subtracting section 102.
A self-aligning torque (SAT) SAT1 detected or estimated in a self-aligning torque section 140 is inputted into a multiplying section 124. A vehicle speed sensitive gain G1 is set on the basis of the vehicle speed V in the vehicle speed sensitive gain section 123. The vehicle speed sensitive gain G1 from the vehicle speed sensitive gain section 123 is also inputted into the multiplying section 124. An output SAT1·G1 from the multiplying section 124 is inputted into a multiplying section 125.
In the meantime, a measured or estimated motor angle velocity ω is inputted into the steering state judging section 120. The steering state judging section 120 judges a steering state, which is steering turning, steering returning or steering holding, on the basis of the steering torque T and the motor angle velocity ω, and inputs a judgment signal as the result of judgment into a steering state sensitive gain section 121.
The judgment of the steering state is, for example, performed according to a flowchart shown in FIG. 3. First, the steering state judging section 120 judges whether the motor angle velocity ω continues to be a same value (or a value within a certain range) for a certain time (Step S100), and the steering state is judged the steering holding when it is judged that it has continued (Step S105). The steering state is judged the steering when it is judged that it has not continued (Step S101), and moreover the steering state judging section 120 judges whether a sign of the steering torque T is identical to a sign of the motor angle velocity ω (Step S102). The steering state is judged the steering turning when it is judged that the sign of the steering torque T is identical to the sign of the motor angle velocity ω (Step S104). The steering state is judged the steering returning when it is judged that the sign of the steering torque T is not identical to the sign of the motor angle velocity ω (Step S103).
The steering state sensitive gain section 121 switches a steering state sensitive gain G2 on the basis of the judgment signal from the steering state judging section 120. In other words, the steering state sensitive gain G2 outputted from the steering state sensitive gain section 121 to the multiplying section 125 is switched according to the judgment signal from the steering state judging section 120. For example, several patterns of combination as follows are possible in the steering state sensitive gain section 121: a pattern (A) of “making the steering state sensitive gain G2 function only in returning a steering wheel” is that the steering state sensitive gain G2 is negative when the steering state is judged the steering returning, the steering state sensitive gain G2 is 0 when the steering state is judged the steering turning, and the steering state sensitive gain G2 is 0 when the steering state is judged the steering holding; a pattern (B) of “making the steering state sensitive gain G2 function only in turning a steering wheel” is that the steering state sensitive gain G2 is 0 when the steering state is judged the steering returning, the steering state sensitive gain G2 is positive when the steering state is judged the steering turning, and the steering state sensitive gain G2 is 0 when the steering state is judged the steering holding; and a pattern (C) of “making the steering state sensitive gain G2 function only in holding a steering wheel” is that the steering state sensitive gain G2 is 0 when the steering state is judged the steering returning, the steering state sensitive gain G2 is 0 when the steering state is judged the steering turning, and the steering state sensitive gain G2 is positive when the steering state is judged the steering holding. These can be summarized as shown in FIG. 4
An output SAT1·G1·G2 from the multiplying section 125 is inputted into a multiplying section 131. Further, a co-sensitive gain G3 (ω) set in a ω-sensitive gain section 130 is also inputted into the multiplying section 131. An output SAT1·G1·G2·G3(ω) from the multiplying section 131 is inputted into a multiplying section 133. A steering torque sensitive gain G4 (T) set in a steering torque sensitive gain section 132 is also inputted into the multiplying section 133. An output SAT1·G1·G2·G3(ω)·G4(T) from the multiplying section 133 is inputted into a multiplying section 135. A steering angle sensitive gain G5(θ) set in a steering angle sensitive gain section 134 is also inputted into the multiplying section 135. A SAT compensation value SATc being an output SAT1·G1·G2·G3(ω)·G4(T)·G5(θ) from the multiplying section 135 is inputted into the subtracting section 102. A subtraction result (Iref−SATc) in the subtracting section 102 is inputted into an adding section 103 as a current command value Iref1, and a compensation signal CM from a compensating section 110 for improving a characteristic is also inputted into the adding section 103.
The compensating section 110 adds an inertia compensation value 111 and a convergence control value 112 in an adding section 113, and inputs the addition result as the compensation signal CM into the adding section 103. An addition result (Iref1+CM) in the adding section 103 is inputted as a current command value Iref2 into a subtracting section 104, and the motor 20 is controlled through a PI control section 105, PWM control section 106 and an inverter 107. The convergence control is braking an action that a steering wheel sways and turns in order to improve convergence of a yaw of a vehicle. For example, it is performed by detecting a change rate of a yaw rate of a vehicle and giving damping to the yaw rate on the basis of the change rate as disclosed in Japanese Published Unexamined Patent Application No. 2000-95132 A.
In this way, the electric power steering apparatus changes a direction of a vehicle by a driver turning or returning a steering wheel. However, there are only two values of turning and returning, and in that case, the two values causes a large variety of chattering, which may give uncomfortable feeling (a torque ripple, a vibration, an abnormal noise, catching or the like) to the driver.